1. Field of the Invention
This invention relates to a bearing and seal assembly for supporting a rotor in the bore of a rotating machine and more particularly to a bearing and seal assembly that provides for adjustment of the rotor into a desired concentricity with the bore.
2. Description of the Prior Art
In rotary machines such as screw compressors, a rotor is rotatably mounted in a housing bore on bearings carried by a bearing support and a gap exists between the rotor and the housing bore. The rotor and housing provide a high pressure cavity and a low pressure cavity. A seal is provided to prevent gas flow from the high to the low pressure cavity.
Installation of a seal in the gap has the effect of creating two separate gaps which will be referred to as (1) the running gap (C1, FIG. 2) which is between the rotor and housing, and (2) the sealing gap (C2, FIG. 2) which is between the interfacing sealing surfaces of the seal and rotor.
The bearing support functions to center and support the rotor in the housing bore. The bearing and seal support affects three parameters which relate to the efficiency of the compressor. First, the width of these two gaps must be as small as possible; second, the width of these two gaps should remain as constant and concentric as possible during operation; and third, the volume of gas flow through the seal must be as small as possible. The prior art has not fully satisfied these three parameters because the components of known bearing and seal assemblies require a plurality of cumulative tolerances that must be accommodated. For example, in a typical bearing support and seal assembly, the components have seven dimensional tolerances which are cumulative. In addition, some components have different thermal expansion and when thermal expansion is also factored in, an overall radial tolerance of 0.010 can exist. In practice, to accommodate this overall tolerance the running and sealing gaps must be slightly increased. However, if the sealing gap is too large and the seal does not properly seal, it will reduce the overall efficiency of the compressor and increase horsepower requirements. Such compressors have horsepower requirements in the 200 hp to 600 hp range and currently annual operating costs for energy are in the range of $300 to $500 per each rated horsepower of the compressor. Therefore, if the gap and seal assembly is more effective, the overall efficiency of the compressor can be improved which means that the horsepower requirements for a given gas output can be reduced resulting in significant savings.
In practice, the seal will be designed to have the smallest possible sealing gap. In an effort to minimize problems, the seal is made of aluminum even though steel or cast iron is preferred. Aluminum improves the self-adjustment qualities of the seal relative the rotor and the sealing gap is designed to be wider than really desired for the highest possible overall compressor efficiency.
During the seal self-adjustment period, the seal may have an important friction contact and such friction can cause the rotor to overheat and expand more than the rotor running gap Cl which may cause the rotor to contact the housing resulting in seizing.
However, aluminum is not the most desirable seal material because the dimension of an aluminum seal is difficult to control during manufacture. The seal dimension will change in response to change in temperature making machining to close tolerances more difficult. In addition, all of the other components on the bearing and seal assembly must be machined to very close tolerances in order to minimize the sum of the concentric tolerances and this further increases manufacturing costs.
Problems also exist with known designs relating to the subassembly of bearing supports and seal components and their subsequent final installation in the rotor housing. The bearing support in prior designs functions to provide concentricity of the rotor with the bore. This need for concentricity requires that the support be precisely machined to provide a close tolerance in the rotor bore. Further, the plurality of very close tolerance components make assembly a very time-consuming and costly process which further adds to the cost of the compressor.